Significant current attention is being given to the development of bridge decks manufactured from fiber reinforced plastics (FRP). One important incentive for this development is the corrosion of conventional steel reinforced concrete which occurs in the presence of water with any salts particularly road salt which tends to infiltrate traditional reinforced concrete bridge decks. Steel reinforced concrete provides the required strength at minimum cost and therefore a major factor in the reluctance to adopt FRP decks is the increased cost which is necessary using conventional techniques to manufacture a deck having the required equivalent strength.
The FRP decks of course have significant advantages over concrete, related not only to the resistance to corrosion, but also the significantly reduced weight which allows installation which can be carried out much more quickly avoiding the necessity for heavy equipment. Also FRP decks increase the live load capacity of the structure by removing heavy concrete decks The inherent advantages of FRP decks have not yet sufficiently overcome the initial cost disadvantages to allow widespread application but some success is now being achieved leading to considerable attention to further development and life cycle cost analysis.
One important current technique which is widely used is that of pre-forming a plurality of pultruded longitudinal sandwich elements which are laminated at the top and bottom with covering sheets so as to form a structural sandwich construction. The pultruded elements can comprise simple tubular bodies or can be more complex with multi-cellular cross sectional construction. Pultrusion necessarily forms a part of constant cross section, as defined by the pultrusion die, and forms outside and inside surfaces which are necessarily accurately flat. Bonding of the top and bottom surfaces of the pultrusion therefore to the laminating sheets can be effected using conventional adhesive materials since the spaces or interstices between the laminating sheets and the pultruded parts are necessarily relatively small so that the thickness of the adhesive is minimized.
The significant disadvantages of pultrusion are however that:
a) The cross section is necessarily constant and therefore it is not possible to modify or tailor the materials within the cross section along the length of the pultruded part to accommodate the different bending and shear forces along the length of the part and bearing forces caused by the loads applied to the panel. PA1 b) Pultrusion necessarily utilizes mat or fabric to provide transverse strength. While some of the fiber reinforcement in the resin material can be provided by rovings, a significant proportion must be provided by fabric or mat and this carries a significantly higher cost due to the intervening manufacturing process. Very approximately, fabric carries a cost of 3 to 4 times the cost of roving or individual filaments thus dramatically driving up the cost of the finished product. PA1 c) The pre-forming of fabrics while dry prior to entering the pultrusion die necessarily forms small folds in the fabric at the corners of the cross-section and these folds in the finished part form flaws which can lead to failure. PA1 collating a structure comprising: PA1 a first sheet of fibers; PA1 a second sheet of fibers generally parallel to and spaced from the first sheet; PA1 and a plurality of intervening connecting members for arrangement between and connection to the first and second sheets, the connecting members being parallel, side by side and extending longitudinally of the panel; PA1 wherein each connecting member is formed from an outer tubular layer defined by a series of helically wrapped filaments wrapped around a core member; PA1 and wherein an unset resin in the filaments of the connecting members is cause to set while the connecting members are in intimate contact each with the next causing the resin and filaments of each connecting member to intermingle with those of the next adjacent member while the resin sets such that the connecting member and the next adjacent connecting member are intimately bonded together by the resin of one being set with the resin of the next and filaments of one being intermingled with filaments of the next.
As the end product is highly cost sensitive, due to the competition from conventional concrete, the above disadvantages of the use of fabric and the inability to minimize material use provides the finished product which is only marginally competitive despite the significant advantages outlined above.
An alternative technique for manufacturing composite panels of this type involves the use of hand lay-up processes in which top and bottom plies of fabric are applied onto pre-formed intervening sandwich members of fabric wrapped foams or balsa wood while both the plies and sandwich members remain in dry condition without the addition of the resin material. From the dry condition, the finished part is formed by infusing resin through the fiber materials so as to integrate the fiber materials into a common structure by the common plastic resin passing throughout the structure.
This technique has the advantage that the structure is formed integrally by the infused resin and thus avoids the necessity for adhesives and bonding. However the structure retains the disadvantage that the lay-up process necessarily involves the use of significant quantities of fabric thus leading to the above cost disadvantages.
The hand lay-up process theoretically allows the cross section of the panel to be tailored along the length of the panel to accommodate the changing loads along the length of the panel but in practice this is very difficult to achieve without the addition of labor to cut and apply variable ply layers of fabric to different sections of the panel.
It will be appreciated that bridge decks of this type are used to span spaced support members or beams so as to transfer the loads from the passing traffic (either pedestrian or vehicular) to the underlying beams.
Without going into the calculation of the forces in detail, it will be appreciated that in general the loads applied to the deck at positions on the deck aligned with the longitudinally spaced supports are substantially at right angles to the deck so as to provide a tendency to shear the deck at the bearing support. The maximum shear loads are therefore at the supports.
In between the supports the shear loads decrease. Loads applied to the deck also tend to bend the panel. The bending forces at the midpoint between the support points are generally the compression in the top sheet and the tension in the bottom sheet for simply supported panels. The loads therefore at the support points are entirely different from those at the midpoints between the support points thus leading to different structural requirements at these different locations along the length of the panel.
U.S. Pat. No. 4,615,166 (Head) issued Oct. 7, 1986 discloses a deck panel of this general type including top and bottom sheets formed of steel and an intervening sandwich member which zigzags back and forth between the sheet members. The sandwich member is formed by pultrusion from fiber reinforced resin material and is bonded to the top and bottom sheets. The spaces in between the sheets are then filled with foam. It is not believed that this construction has achieved significant commercial success.
A more recent structure is shown in U.S. Pat. No. 5,794,402 (Dumiao et al) issued Aug. 18, 1998. This shows a modular bridge construction which preferably incorporates beams formed from FRP which are attached to an overlying deck panel construction.
The deck panels are formed again from top and bottom sheets together with intervening preformed cured tubular members which are arranged side by side along the panel and bonded to the sheets.
Again the tubular members are preferably formed by pultrusion from fiber reinforced resin and in practice the products manufactured by the assignee of this patent (Martin Marietta Materal Inc) have used large fully cured single-cell pultrusions between the top and bottom sheets.
The patent does mention in passing and purely speculatively that the tubular members may be formed by other techniques such as "hand lay-up or other suitable methods including resin transfer molding (RTM), vacuum curing and filament winding, automated lay-up methods and other methods known to one of skill in the art of composition fabrication". However it is believed that these techniques have not been used in practice and no detail is provided as to how these techniques might be employed. In addition the patent describes first the complete forming of the tubes indicating that the method uses a cured structural member.
As set forth above the pultrusion method for the formation of the intervening members between the two sheets necessarily includes the use of expensive fabric thus significantly increasing the cost of the finished product.
Another process is described in a paper published at the 31.sup.st International SAMPE Technical Conference Oct. 26.sup.th to 30.sup.th 1999 by Aref et al and discloses a technique in which cured filament wound cores of oval shape are arranged side by side and wrapped by a further layer of filament winding. The structure is then infused with resin and the resin cured. However this proposal again uses cured cores so that the bonding of each to the next is limited by the strength of the resin bond and there is no filament intermingling. The use of oval cores in any event provides little contacting surface area.